Concrete is commonly mixed in batches in a drum batch mixer, including tilting and non-tilting drum mixers.
One typical configuration of a non-tilting reversing drum mixer includes a horizontally oriented cylindrical drum having an inlet opening at a first end for charging the mixer with concrete materials to be mixed, an outlet opening at an opposing second end for discharging mixed concrete from the drum, and a plurality of mixing paddles supported in the drum to extend generally radially inwardly from the cylindrical wall of the drum for mixing concrete as the drum is rotated. The mixing paddles are oriented to urge the concrete materials towards the inlet opening when the mixing drum is rotated in a first mixing direction and to urge the concrete materials towards the outlet opening when the mixing drum is rotated in a second discharging direction.
Alternatively, in some instances of non-tilting drum mixers, the inlet opening and discharge opening may be located at the same end of the drum. In this instance the mixing paddles are oriented to urge the concrete materials away from the discharge opening when the mixing drum is rotated in a first mixing direction and to urge the concrete materials towards the outlet opening when the mixing drum is rotated in a second discharging direction.
A known desirable feature of reversing drum mixers is that the mixing paddles are substantially self-cleaning after each batch of concrete being mixed because the mixing paddles are typically specifically designed with smooth surfaces terminating at smooth continuous inner free edges while being oriented at a relative steep inclination of approximately 35-45 degrees to the longitudinal axis of the drum. This paddle configuration ensures that the concrete materials readily slide off of the paddles during mixing and discharging.
In tilting drum mixers, the drum axis is typically oriented near horizontal for mixing with paddles oriented to urge material away from the discharge opening at the discharge end of the drum to retain the concrete materials in the drum while mixing. The drum is then tilted so that the drum axis is oriented at a downward inclination towards the discharge opening for discharging. It remains desirable for the mixing paddles to have smooth surfaces to avoid excess accumulation of concrete materials thereon when discharging.
Occasionally when mixing high slump concrete using typical mixing paddles as described above, pockets of unmixed material remain in the concrete materials being mixed. These pockets of unmixed material are difficult to break up due to the high fluidity of the mixture and due to the design of the paddles which are intended to be substantially self-cleaning with smooth edges, smooth surfaces, and sharp inclinations relative to the longitudinal axis of the mixer as noted above with regard to reversing drum mixers for example.